“Vertical GaN p-n Junction Diodes With High Breakdown Voltage Over 4 kV”, Hiroshi Ohta et al. IEEE ELECTRON DEVICE LETTERS, Vol. 36, No. 11 (2015). pages 1180 to 1182 describes a technique that divides an n-type drift layer being a voltage-blocking layer into two or more layers. and sets a donor concentration in the n-type drift layer in contact with a p-type layer lower than a donor concentration in an n-type drift layer that is not in contact with the p-type layer. This configuration allows an electric field generated in a pn junction between the p-type layer and the n-type drift layer to be alleviated.